[TG] [TG11] Placing a waypoint on the sounder screen

[merlin956]

I marked a waypoint from the sounder screen yesterday and I was surprised to see that the point was off to the right of my track.
Referring to the attached picture I placed the skull and crossbones mark on the structure shown on the waypoint. The marker appeared on the GPS screen well to the right of my current track. By the time I took the screenshot, the boat had passed the structure but when I marked it the angle between the boats track and the marker was approximately 90 degrees.
As it turns out the marker was placed exactly over the structure. I made a pass back to check,
As a reference, the distance between the red cross and the rocks symbol is approximately 40 ft, which means the skull and crossbones is about 30 ft off my beam. The water depth was about 60 ft.
As I understand sounder operation it has no idea where in the cone an object is, just that it is somewhere in the cone.
What I don't understand is that the sounder and down vision show what is under the boat. So if the mark was placed on the structure shown on the sounder screen and the mark is on the structure, how could the boat's track be so far away?

[Tom - Raymarine - Moderator]

Hello merlin956,

Quote:...how could the boat's track be so far away?

There are two aspects to the answer I'd give to that:

1. GPS position fixes are never 100% accurate, there is always some degree of inaccuracy and drift in your calculated position fix over time, and if your GPS fix is less accurate than normal for one of a range of reasons then you will get position errors that could easily be of that kind of distance.
2. It looks like you may have a relatively low Track sampling rate set. In the default Auto track setting, the system will only plot a track point when there's a sharp turn in your course, and if you're making smaller, more gradual course changes then the system won't necessarily leave a new track point. In this instance the track line 'follows' the boat, always showing a straight line between the current boat position and the last track point. This can make it look like you've passed in a slightly different course to what you actually have, at close range scales. To avoid this go into Track setup and change from Auto to maybe a time-based track type and set a short-ish sampling interval. It's a trade-off between the smoothness and accuracy of the track and the length of time we can record for without running out of memory, which is why the Auto setting drops fewer track points.
   

Regards,
Tom

[merlin956]

I think the GPS fix is fairly accurate as I can go back to the mark and find the structure.
I think part 2 of your answer hits the nail on the head. I hadn't considered the impact that the behaviour of the tracking algorithm would have. What you say makes perfect sense.
Thanks for clearing it up.